Many precision systems, such as those used in semiconductor processing, inspection and testing, use linear motors for positioning objects such as semiconductor wafers. Such a system (e.g. a lithographic instrument) may have an X stage stacked on a Y stage. Conventional systems are complex, heavy and inefficient to operate. In order to improve precision and efficiency of object positioning, planar motors are designed for simplicity, light weight, and better efficiency.
In accordance with the Lorentz law, linear or planar motor uses electromagnetic force (frequently called Lorentz force) to propel a moving part. As those skilled in the art will recognize, a wire carrying an electric current in a magnetic field creates Lorentz force, the formula of which is: EQU F=N L B.times.I,
where F represents Lorentz force, N the number of wires, B the magnetic flux, and I the electric current. For a coil with a given length L and magnetic flux B, to maximize force F, one has to maximize the number of wires N and current I. The above formula determines both the magnitude and the direction of force F, since force F, magnetic flux B, and current I are all represented as vectors, and the symbol "x" represents vector cross product multiplication. Accordingly, force F is directed perpendicular (orthogonal) to the plane defined by magnetic flux B and current I, and is maximized when magnetic flux B and current I are directed orthogonally relative to each other.
Disclosures in the field of planar motors include Hinds U.S. Pat. No. 3,851,196, Hinds U.S. Pat. No. 4,654,571, Trumper U.S. Pat. No. 5,196,745, and Chitayat U.S. Pat. No. 5,334,892. These patents describe planar motors that have significant limitations. For example, the planar motor of Hinds '196 has limited range of motion because each portion of the stationary magnet array can only generate force in a single direction. Thus, each coil array must always be located above the corresponding magnet array. This limits the range of movement for a given size actuator. In addition, the coils and magnets are iron-core and generate sizable attractive forces as well as force ripple. This does not allow for motion in six degrees of freedom because the levitation force cannot overcome the attractive force between the two pieces.
Hinds '571 suffers from a non-compact design. A large portion of the base of the moving portion of the stage is covered by the air bearing pads and other elements. Only a small portion of the stage is covered with coils. In addition, the coil design is not the most efficient for producing force, since at most only fifty per cent of the coil can generate force. In addition, the moving coil design has a large number of hoses and cables going to the stage, creating a large bias force. Finally, this design does not generate force for a six-degree-of-freedom movement.
Trumper discloses several stage designs with six degrees of freedom. The invention uses conventional coils. Each coil array must be located above a corresponding linear magnet array. This restricts the range of movement for a given sized stage.
Chitayat discloses several planar motor designs, which permit a wide range of motion, but only restricted to translation and rotation in a plane. Thus, the motor of Chitayat is incapable of moving with six degrees of freedom.
What is needed in the art is a planar motor that provides a wide range of motion in six degrees of freedom with high speed and precision, having a compact configuration and energy-efficient operation without cumbersome hoses and cables attached to the moving stage.